1. Field of Invention
The present invention relates to a membrane separation device suitable for water treatment, such as sewage treatment and wastewater treatment, which employs an activated sludge process.
2. Description of the Related Art
Conventionally, sewage treatment and wastewater treatment employing the activated sludge process use a submerged membrane separation device for a liquid-solid separation which is necessary in the treatment process.
For example, Japanese Laid-Open Patent No. JP H07-256282A discloses a membrane separation type small-sized wastewater treatment tank provided with a built-in membrane separation device having a plurality of membrane elements. Each of the membrane elements includes a membrane support body having a plate or sheet-like shape and a separation membrane for filtration disposed on each of the front and rear surfaces of the membrane support body. The plurality of the membrane elements are arranged in a longitudinal position with a fixed interval such that the separation membranes of the membrane elements are facing one another and each of the membrane elements is supported with a side portion thereof in a transverse direction.
Each of the membrane elements has a suction nozzle connected to a collection pipe, where each suction nozzle leads out treated water which has been filtered through the separation membrane. Filtration operation is performed by operating a suction unit such as a pump such that the treated water is sucked via the collection pipe.
Such a membrane separation device is provided with a diffuser at a lower part thereof. An air-lift action of air bubbles supplied from the diffuser causes an upward flow in a distance between the membrane elements in which the water to be treated goes up together with the activated sludge, and the upward flow of this gas-liquid mixed phase provides aeration cleaning of the membrane surface of the separation membrane.
Such aeration cleaning prevents deterioration of the membrane-separation function caused by fouling. Also, it is known that the cleaning effect of the separation membrane is enhanced by carrying out aeration from the diffuser in a state where the filtration operation by the separation membrane is stopped.
However, as shown in FIG. 10A, a bonding portion 83 in which the membrane support body 81 and the separation membrane 82 are joined each other along the longitudinal direction is formed on both sides of the membrane element 80 in the transverse direction.
As shown in FIG. 10B, if the aeration by the diffuser is continued when the filtration operation is stopped, the treated water collected between the membrane support body 81 and the separation membrane 82 and a gas component which has been dissolved into the treated water and then evaporated when filtered through the separation membrane 82 are pushed upward by the upward flow caused by the air-lift action, so as to form a big bulging portion 84 in an upper part of the membrane element 80. As a result, the bulging portion 84 acts as resistance against the upward flow so as to cause vibrations in the separation membrane 82.
An amount of the bulge of the separation membranes 82 bulging into a space between the adjacent membrane elements 80 which are arranged to have the fixed distance therebetween is restricted when the bulging portions 84 touch each other. However, since there is no separation membrane to restrict the bulge of the separation membrane 82 between the membrane element 80 which is disposed at the outermost end along the arrangement direction and a cover member 85 which is arranged outside of the outermost membrane element 80 with the same interval, the amount of the bulge of the separation membrane 82 of the outermost membrane element 80 becomes large.
As a result, an angle between the membrane support body 81 and the separation membrane 82 in the vicinity of the bonding portion 83 on the outer side of the outermost membrane element 80 becomes larger than an angle between the membrane support body 81 and the separation membrane 82 in the vicinity of the bonding portion 83 on the inner side of the outermost membrane element. Consequently, a large force is applied to the separation membrane 82 in the bonding portion 83, causing a problem that there is a risk that the separation membrane 82 may rupture.